1. Field of the Invention
This invention relates to a measuring device for measuring the three-dimensional shape of an object in a non-contact type manner by use of an optical probe.
2. Description of the Related Art
Conventionally, various types of optical shape measuring devices have been proposed. There are two types of measurement principles of the optical shape measuring devices: one type utilizing the geometric optical property of light and the other type utilizing the wave optical property of light. Further, the measurement method includes a passive method of detecting a reflected light from an object in the natural light and an active method of applying light to an object by using a special light source and detecting a reflected light from the object.
When the shape of a spherical surface is measured, the active method utilizing the geometric property of light can be adequately used. This is because it is generally required to determine measurement points when the shape of the spherical surface is measured. Therefore, a method of focusing an optical beam on a measurement point and then determining the position based on information of light reflected from the measurement point is used.
An optical probe (including a semiconductor laser and an optical position detecting sensor) represented by a laser scanning probe of RENISHAW utilizes the trigonometrical survey as a basic principle. This type of optical probe is practical in the measurement precision and the measurement range, but since it has the shadow effect, the tilt angle of a to-be-measured object which can be measured by use of this optical probe is limited. For this reason, various methods for attaining enhancement of the measurement precision and reduction in the shadow effect are proposed. However, in the sensor based on the trigonometric survey, the influence by the shadow effect cannot be prevented if the light projection angle is fixed. Further, a range finder using a CCD camera is proposed as a sensor of measuring the entire shape of an object at a high speed. In this case, however, the influence by the shadow effect cannot be eliminated and it is not satisfactory as a shape measuring sensor from the viewpoint of the measurement precision.
Several methods using an optical fiber as an optical distance sensor are proposed. For example, such methods are disclosed in the following articles:
G. Hull-Allen: Reflectivity Compensation and Linearization of Fiber Optic Proximity Probe Response, SPIE, Vol.518, Optical System Engineering, 1984, p 81; PA1 L. Hoogenboom, G. Hull-Allen, S. Wang: Theoretical and Experimental Analysis of a Fiber Optic Proximity Probe, SPIE, Vol.478, Fiber Optics and Laser Sensors, 1984. p 46; PA1 H. Kopola, S. Nissila, R. Myllyla, P. Karkkaisen: Intensity Modulated Fiber Sensor for Robot Feedback Control in Precision Assembly, SPIE, Vol.798, Fiber Optic Sensors, 1987, p166; PA1 G. Conforty, M. Brenci, A. Mencaglia, A. G. Mignani, A. M. Scheggi: Optical Fiber Sensor for Vibration Monitoring in High Power Electrical Plants, SPIE, Vol.1011, Fiber Optic Sensors, 1988, p 116; PA1 F. C. Cuomo: The Analysis of a Three-Fiber Lever Transducer, SPIE, Fiber Optic and Laser Sensor, 1984, p 29; and PA1 L. Xiaoming, R. Xin, W. Peizheng, C. Rongsheng: Reflective Optical Fiber Displacement Sensor, SPIE, Vol. 1572, International Conference on Optical Fiber Sensors in china, 1911, p 248. PA1 H. Bukow: Fiber Optics Distance Sensor for Robotic Application, Technical Paper of SME Conference (Sensor 86, Detroit, Michigan), 1986; and PA1 H. Bukow, M. Bailey, W. Stevenson: Simulation of Reflectance Sensors Using Image Synthesis Techniques, Computers in Mechanical Engineering, January 1985, p 69.
The above methods are effected to measure a distance based on an amount of reflected light obtained by applying a light beam from an emitter fiber to a to-be-measured object and detecting the reflected light by use of a detector fiber. Further, the above methods are effective when the inclination of the surface of the object, the intensity of light from the light source and surface reflectance characteristic (surface diffusion characteristic) of the surface of the object are constant. However, the measurement of shape including the inclination of the object surface is difficult.
Therefore, H. Bukow et al. have proposed a method of measuring a distance based on a difference between amounts of light in two detector fibers which are provided for the emitter fiber in order to reduce the influence by the inclination of the object surface, the intensity of light from the light source and the surface reflectance characteristic of the object surface. For example, refer to the following articles:
In their experimentally manufactured probe, in order to reduce the influence of the shadow effect and enhance the measurement precision, four sets of detector fibers each set including two detector fibers are arranged at a 90 deg. interval around the emitter fiber. With the experimentally manufactured probe, variations in the inclination of the surface of the object, the intensity of light from the light source and surface reflectance characteristic of the surface of the object can be offset to some extent. However, when the inclination of the object surface exceeds 30deg., it becomes difficult to detect the reflected light. Further, in their above paper, only the distance measurement using the experimentally manufactured probe is referred to and the normal detection method for the measurement points of the object is not disclosed. In order to develop an automatic high-speed learning control system, it is not sufficient for the sensor to have the distance detection function but the unit normal vector detection function must be provided.
For example, R. Shoureshi et al. have constructed a learning control system by attaching a distance sensor proposed by H. Bukow et al. to the front end of a robot hand (refer to R. Shoureshi, R. Evans, W. Stevenson: Optically Driven Learning Control for Industrial Manipulators, IEEE Control Systems Magazine, October 1989, p 21). However, this system is designed for automatic brushing, painting, debugging, welding and seam tracking and is not required for enhancing the positioning precision of the robot hand and increasing the positioning speed. Further, the above system is designed to deal with a curved surface having small variations in the gradient and curvature thereof. T. Miyoshi et al. tried to effect the high-speed learning measurement for the curved surface by setting a distance sensor based on the trigonometric survey on the main shaft of a machine tool (for example, refer to T. Miyoshi, T. Kondo, K. Saito, Y. Kamiya, H. Okada: Development of Non-Contact 3-D Digitizing System, Journal of JSPE, Vol.56, No.6, 1990, p 1021). In this case, however, since the learning measurement using only distance information is effected, there occurs a limitation of possibility for enhancing the control speed and smooth controllability.